An extensive prior art exists for the manufacture of chromium-free conversion layers on the abovementioned metal surfaces.
U.S. Pat. No. 5,129,967 discloses treatment baths for a no-rinse treatment of aluminum (described as “dried in place conversion coating”), comprising
a) 10 to 16 g/l polyacrylic acid or homopolymers thereof,
b) 12 to 19 g/l of hexafluorozirconic acid,
c) 0.17 to 0.3 g/l of hydrofluoric acid and
d) up to 0.6 g/l of hexafluorotitanic acid.
EP-B-8 942 discloses treatment solutions, preferably for aluminum tins, comprising
a) 0.5 to 10 g/l polyacrylic acid or an ester thereof and
b) 0.2 to 8 g/l of at least one of the compounds H2ZrF6, H2TiF6 and H2SiF6, wherein the pH of the solution is below 3.5.
Additional polymers that can be used in similar treatment baths are listed in WO 02/20652.
U.S. Pat. No. 4,992,116 describes treatment baths for the conversion treatment of aluminum at pH values between about 2.5 and 5 and which comprise at least three components:    a) phosphate ions in concentrations between 1.1×10−5 to 5.3×10−3 mol/l corresponding to 1 to 500 mg/l,    b) 1.1×10−5 to 1.3×10−3 mol/l of a fluoric acid of an element of the group Zr, Ti, Hf and Si (corresponding to 1.6 to 380 mg/l, depending on the element) and    c) 0.26 to 20 g/l of a polyphenolic compound, obtained from the reaction of polyvinyl phenol with aldehydes and organic amines.
WO 92/07973 teaches a chromium-free treatment process for aluminum, which uses 0.01 to about 18 wt. % H2ZrF6 and 0.01 to about 10 wt. % of a 3-(N—C1-4 alkyl-N-2-hydroxyethylaminomethyl)-4-hydroxystyrene polymer in acidic aqueous solution as the major components. Optional components are 0.05 to 10 wt. % dispersed SiO2, 0.06 to 0.6 wt. % of a solubilizer for the polymer as well as surfactant. The cited polymer falls in the group described below of suitable “reaction products of polyvinyl phenol with aldehydes and organic amines containing hydroxy groups” in the context of the present invention.
WO 94/28193 describes a process for the pretreatment of aluminum surfaces or its alloys prior to a second, permanently corrosion-protective conversion treatment, wherein the surfaces are contacted with acidic, aqueous treatment solutions that comprise fluorine complexes of the elements boron, silicon, titanium, zirconium or hafnium alone or in mixtures with fluorine anion concentrations totaling between 100 and 4000, preferably 200 to 2000 mg/l and have a pH between 0.3 and 3.5, preferably 1 and 3.
Thereby the treatment solutions may comprise additional polymers of the type of polyacrylates and/or the reaction products of polyvinyl phenol with aldehydes and organic amines containing hydroxy groups in concentrations below 500 mg/l, preferably below 200 mg/l. Further possible components of the treatment baths are polyhydroxycarboxylic acids or their anions, particularly gluconate ions, in concentrations up to 500 mg/l.
WO 95/04169 teaches the treatment of metal surfaces with a treatment solution that comprises the following components: Fluoro complexes of titanium, zirconium, hafnium, silicon, aluminum and boron, metal ions selected from cobalt, manganese, zinc, nickel, tin, copper, zirconium, iron and strontium, phosphates or phosphonates as well as water-soluble or water-dispersible organic film formers.
WO 00/71626 discloses a chromium-free anti-corrosive, comprising water and    a.) 0.5 to 100 g/l hexafluoride anions of titanium (IV), silicon (IV) and/or zirconium (IV)    b.) 0 to 100 g/l phosphoric acid    c.) 0 to 100 g/l of one or a plurality of compounds of cobalt, nickel, vanadium, iron, manganese, molybdenum or tungsten.    d.) 0.5 to 30 wt. % of at least one water-soluble or water-dispersible film-forming organic polymer or copolymer (based on the active substance),    e.) 0.1 to 10 wt. % of an organophosphonic acid    f.) optional additional auxiliaries and additives.
In the abovementioned documents, in many cases it is obvious that the produced conversion layers are colorless and transparent, such that the treated metal surfaces appear metallic bright. At least, it was never disclosed in these documents that colored structures were obtained. However, from many years of experience with chromating, the expert is accustomed to obtaining a colored layer from the conversion treatment. He or she can then immediately recognize visually whether the treatment has produced the desired success. When manufacturing colorless layers, however, a more complex surface analysis is required, for example the determination of the Ti content of the surface by means of an X-ray fluorescence measurement. There is therefore a practical need for surface treatment processes that not only provide similarly good anti-corrosion properties and paint adhesion as with conventional chromium-containing layers, but also layers, similar to those chromium-containing layers, which are visible to the human eye.
Approaches for solving this problem exist in the prior art. For example, WO 94/25640 discloses a process for the production of blue-colored conversion layers on zinc/aluminum-alloys. The metal surfaces are contacted with a treatment solution at a pH between 3.5 and 6 with a 0.2 to 3.0 wt. % molybdenum content and a fluoride content of 0.1 to 2.0 wt. %. Molybdenum can be used in the form of molybdate, as phosphomolybdenic acid, as molybdenum chloride and such. Fluoride can be used in the form of hydrofluoric acid, simple fluorides, but also as complex fluoro acids such as for example fluorotitanic acid or fluorozirconic acid.
A similar treatment process is described in WO 95/14117, although no mention is made there of the optical appearance of the conversion layers. According to this document, the treatment solution comprises heterooxo anions of molybdenum, tungsten or vanadium with one of the heteroions phosphorus, aluminum, silicon, manganese, zirconium, titanium, cerium or nickel. Moreover, the treatment solution comprises an organic film-builder, which for example, can be selected from acrylates. The heterooxo anions, for example anions of heteropoly acids, can be formed directly in the treatment solution, in which one adds the starting products for them, for example molybdate ions and phosphoric acid. In addition, the treatment solution should preferably comprise a corrosive for aluminum, e.g. fluoride, tetrafluoroborate or similarly acting corrosives.
The teaching of WO 00/26437 follows the path of dyeing the conversion layer with an organic dye (alizarin dye). The conversion layer itself is produced with a treatment solution that comprises complex fluorides, for example of titanium and zirconium besides additional inorganic oxides, hydroxides or carbonates or their reaction products with the fluoric acids. In addition, poly-4-hydroxystyrene (polyvinyl phenol), substituted with amino groups, can be present as the organic polymer.